The present invention relates to an improved apparatus for controlling an A-C powered elevator.
FIG. 1 illustrates a conventional apparatus which employs an induction motor to drive the cage of an elevator, the induction motor being supplied with an A-C power having a variable voltage and a variable frequency.
In FIG. 1, symbols R, S, T denote a three-phase A-C power source, reference numeral 1 denotes a rectifier, numeral 2 denotes a capacitor for smoothing the D-C output of the rectifier 1, numeral 3 denotes an inverter of the well-known PWM type system which is connected to the D-C side of the rectifier 1 and which converts a constant D-C voltage into an A-C power having a variable voltage and a variable frequency which is controlled by pulsewidth modulation control, numeral 4 denotes a three-phase induction motor powered by the inverter 3, numeral 5 denotes a brake wheel coupled to the motor 4, numeral 6 denotes a brake shoe which comes into engaging and disengaging contact with the periphery of the brake wheel 5 and which applies a braking force when it engages therewith, said brake shoe being forced against said brake wheel by a spring (not shown). Numeral 7 denotes a brake coil, which when energized, pulls the brake shoe 6 away from the brake wheel 5 thereby overcoming the force of said spring, numeral 8 denotes a drive sheave of a hoist which is driven by the electric motor 4, numeral 9 denotes a main rope wound around the sheave 8, numeral 10 denotes a cage connected to the main rope 9, numeral 11 denotes a balance weight, numeral 12 denotes an inverter for regenerating electric power and which is connected between the power source terminals R, S, T and the output side of the rectifier 1, numeral 13 denotes a transformer of which the primary side is connected to the power source R, S, T, numeral 14 denotes a rectifier which is connected to the secondary side of the transformer 13, numeral 15 denotes a start/stop device which is connected to the D-C side of the rectifier 14 and which is utilized to start and stop the cage 10, numeral 16 denotes a brake controller which is connected to the brake coil 7 for the control thereof, numeral 17 denotes a contactor controller, numeral 18 denotes an electromagnetic contactor which is controlled by the contactor controller 17, i.e., which is energized when the cage 10 is to be started and de-energized when the cage 10 is to be stopped, reference numerals 18a to 18c denote normally-open contacts connected to the rectifier 1 on the side of the power source, numeral 19 denotes an electromagnetic contactor which is energized after the electromagnetic contactor 18 is energized and which is de-energized after the electromagnetic contactor 18 is de-energized, reference numerals 19a to 19c denote normally-open contacts connected to the output side of the inverter 3, numeral 20 denotes a speed controller which controls the inverters 3 and 12, and numeral 21 denotes a door controller for opening and closing the door of the cage 10.
The operation of the system is as described herebelow.
When the cage 10 is at rest, the brake shoe 6 is pressing against the brake wheel 5 due to a pressure of a spring (not shown). When the cage 10 is instructed to move, the electromagnetic contactor 18 is energized, which causes the contacts 18a to 18c to close, thereby applying the A-C power source to the rectifier 1 which, in turn, produces a D-C output. Consequently, the capacitor 2 is electrically charged. As the voltage of the capacitor 2 reaches a predetermined value, controls elements (not shown) in the inverter 3 are successively rendered conductive whereby A-C power of a variable voltage and variable frequency is produced maintaining a phase sequence that corresponds to the direction of operation of the elevator. Then, the electromagnetic contactor 19 is energized, which causes contacts 19a to 19c to close, and the A-C power from the inverter 3 to be supplied to the electric motor 4. At the same time, the brake coil 7 is energized, and the brake shoe 6 disengages from the brake wheel 5. Thus, the electric motor 4 rotates in a direction determined by the phase sequence inputted from the speed controller 20 to inverter 3, and the cage 10 commences to operate. The speed controller 20 operates, the output frequency of the inverter 3 is adjusted according to speed instruction signals from the speed controller 20, thereby controlling the running speed of the motor 4, i.e., the running speed of the cage 10 is controlled. The speed can be controlled in a variety of ways; i.e., voltage/frequency constant control method, slip frequency control method, vector control method, and the like. These methods, however, are not described herein.
During the so-called load-raising operation such as when the cage is accelerating in an upward direction with a heavy load or in a downward direction with a light load, the electric power is supplied to the motor 4 via the inverter 3. During the so-called load-lowering operation such as when the cage is decelerating in an upward direction with a light load or in a downward direction with a heavy load, the motor 4 assumes the state of a regenerative operation, and the regenerated electric power flows into the inverter 3 and stored in the capacitor 2. However, during the load-lowering operation the voltage may rise excessively and destroy elements in the inverter 3. Therefore, the power regenerating inverter 12 is utilized for returning the regenerated electric power back to the A-C side thereby protecting the inverter 3.
However, provision of the power regenerating inverter 12 inevitably increases the cost of the system. It has also been attempted to consume the regenerated electric power through resistors. According to this method, however, the electric power is wastefully consumed, which is not desirable from the standpoint of saving energy.